1. Field of the Invention
This invention generally relates to laser scanning systems for reading bar code symbols and, more particularly, to various optical systems for changing the working distance and/or the reading spot size of an outgoing laser beam scanned across the symbols to be read. This invention also relates to systems for turning oval-shaped outgoing laser beams.
2. Description of the Related Art
Laser scanning systems and components of the type exemplified by U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,673,805; 4,758,717; 4,760,248; 4,736,095; 4,460,120 and 4,607,156--all of said patents being owned by the assignee of the instant invention and being incorporated by reference herein--have generally been designed to read bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working or reading distance from a hand-held or stationary scanner, and with a reading spot of a certain size. The particular spot size and working distance are typically optimized in dependence upon the particular application and, in effect, the system tends to be custom-made for each intended use.
For example, UPC symbols are typically affixed on objects in at least three different densities or sizes, depending to a great extent on the size of the object itself. So-called "high-density" (HI-D) symbols are typically characterized by very thin bars separated by very thin spaces and, hence, are typically affixed to small objects. So-called "low-density" (LO-D) symbols are generally characterized by very broad bars separated by very broad spaces and, hence, are typically affixed to large objects. So-called "medium-density" (MED-D) symbols are generally characterized by bars and spaces whose respective widths along the scanning direction lie somewhere between those of HI-D and LO-D symbols, and are affixed to medium-sized objects. The definitions of HI-D, LO-D and MED-D symbols in terms of numerical values may be different for different applications, bu for any one particular application, e.g. the inventorying and check-out of supermarket goods, these relative definitions and their numerical values are readily understood by those skilled in the art.
To read HI-D symbols with accuracy, a very fine reading spot, e.g. a six mil diameter circular spot, is desired. The known optical systems for forming such a very fine spot produce a very highly divergent laser beam and, as a result, the working distance is correspondingly very short. To read LO-D symbols with accuracy, a very large reading spot, e.g. a forty mil diameter circular spot, is desired. The known optical systems for forming such a very large spot produce a laser beam with very low divergence and, as a result, the working distance is correspondingly very long.
Hence, it will be appreciated that no single known laser scanning system can read both LO-D and HI-D symbols, because the known optical systems designed to read LO-D symbols cannot read HI-D symbols, and vice versa. An optical system designed to read LO-D symbols will have a very long working distance--which is very desirable to read close-in and far-out symbols--but the very large spot size will simultaneously overlap at least one bar and its adjacent space, thereby obscuring a HI-D symbol. On the other hand, an optical system designed to read HI-D symbols has a very short working distance which is very disadvantageous to read far-out symbols. It would be desirable to combine the very long working distance characteristic of LO-D reading systems with the very fine spot size characteristic of HI-D reading systems in a single instrument.